Charting the Real Toll of Bird Flu

Scientists are racing to understand the impact of bird flu and whether certain wildlife, from condors to marine mammals, will be able to fight back

By Katarina Zimmer
Wildlife Science
Dec 17, 2025

A scientist collects dead Humboldt penguins from a beach in Camaná, Peru, in June 2023. The carcasses were tested for the H5N1 virus that causes bird flu. (Photo: ©Víctor Gamarra-Toledo)

IN MARCH 2023, Tim Hauck realized something was wrong. For more than a week, a young female California condor known as 982 had been acting strangely. On a cliff ledge in Arizona’s Vermilion Cliffs National Monument, she sat without moving much—a symptom of lead poisoning. But when Hauck, the California condor program director at The Peregrine Fund, and his colleagues lured the bird into a trap with a calf carcass, her blood tested relatively low for lead.

The team began to worry that 982 had contracted the highly pathogenic H5N1 virus, a deadly form of bird flu responsible for a recent string of domestic and wild bird die-offs around the globe. The virus could be catastrophic for California condors, a federally endangered species numbering fewer than 350 individuals in the wild.

Even before 982’s test results confirmed she had the virus, other condors in the population began dropping dead. Many had neurological problems, their bodies contorted in odd ways or unable to move and falling off cliff faces. “We lost 21 birds in just three weeks,” Hauck says, including many breeding individuals that are key to keeping the population going.

Hauck reckons the toll set back conservation efforts for the 100-strong Arizona–Utah population by about a decade or more—tough news for a species only recently nursed back from the brink of extinction. “We have personal relationships with every single bird. We know their ins and outs and their personal behaviors,” he says. “The hardest part of the whole incident was that, really, there’s not a lot you can do.”

Since late 2020—as COVID-19 was spreading rapidly among humans—a wildlife pandemic has been raging worldwide. While COVID-19 no longer is considered a public health emergency, H5N1 continues to spread and kill. According to the United Nations Food and Agriculture Organization, more than 500 species of wild birds have been infected—from pelicans in Peru and cormorants in South Africa to cranes in Japan and skuas in Antarctica. In the United States, infections have been documented in every state. Though nobody knows the true toll, a 2025 study published in Nature Reviews Biodiversity states that “multiple millions of wild animal individuals may have died.”

Many wildlife scientists, who are just beginning to grasp the scale of the pandemic’s impact, worry about potentially serious, long-term implications for wildlife populations. Fortunately, they also are finding reasons for hope. To help protect some of the most vulnerable animals, new vaccines are being tested, while scientists are learning that many species’ immune systems can fight off the virus on their own. “I think wildlife populations can be a lot more resilient than we give them credit for,” says David Stallknecht, a wildlife disease expert at the University of Georgia’s College of Veterinary Medicine. “And I hope that is a true statement,” he adds.

The endangered California condor has been hit hard by the bird flu pandemic, a blow to recovery efforts that only recently brought the species back from near extinction.

A virus takes hold

Avian influenza viruses long have been evolving and circulating in wild bird populations around the globe, with some of them infecting and killing poultry. The majority of variants never cause any serious illness. But in 1996, scientists identified an especially deadly bird flu variant—the highly pathogenic H5N1 virus—in commercial geese in southeastern China. While most avian influenza viruses multiply mainly in birds’ intestines, mutations gave the 1996 variant H5N1 the ability to infect multiple organs, including the brain, which is why many infected birds have neurological symptoms.

Until the early 2010s, H5N1 was only a problem for chickens and other poultry, causing die-offs—and billions in lost revenue for poultry farmers—across Asia, the Middle East and parts of Africa. Most outbreaks in wild birds fizzled out, says University of Hong Kong virologist Vijaykrishna Dhanasekaran. But by 2021, the virus had begun spreading voraciously among both domestic and wild birds everywhere, proving especially deadly once it reached Newfoundland in Canada in late 2021 then rapidly moved down the Americas.

Some of the most devastating mortality estimates come from South America. According to the Nature Reviews Biodiversity study, more than 47,000 Peruvian boobies, 29,000 guanay cormorants and 2,700 Humboldt penguins have perished. “This was terrible: masses of dead birds in the beaches and dying with neurological signs,” says wildlife ecologist and study co-author Pablo Plaza of Argentina’s National Scientific and Technical Research Council.

Bird species that prey on or scavenge infected waterfowl—including bald eagles in the United States—also have proven particularly vulnerable, says Nicole Nemeth, a wildlife pathologist who heads the Southeastern Cooperative Wildlife Disease Study’s Research and Diagnostics Service. In early 2022, she and her colleagues started receiving dead eagles from Florida and Georgia. People had reported the birds shaking or twitching their heads, having seizures or seemingly paralyzed and dropping dead off perches or out of nests.

Cutting the dead birds open, Nemeth and her colleagues saw vast amounts of damage across their digestive systems, livers and spleens. “If they’re eating a dead duck that died of highly pathogenic avian influenza virus, they’re getting a huge dose of virus in a short period of time,” she says. “There’s just a lot of damage very rapidly, and not a lot of time for the body to respond or defend itself.” Nemeth reckons the service received more than 100 eagles that year, likely making a significant dent in the species’ nesting success in some regions by killing off parents and, as a consequence, their eggs or nestlings.

An image of two southern elephant seal pups.

The H5N1 virus also has taken a heavy toll on the southern elephant seal. In Argentina, more than 17,000 elephant seal pups died in 2023 alone.

Deadly spread to mammals

Experts have been particularly shocked to see the bird-targeting virus become adept at infecting mammals. According to a 2025 tally published in the journal Influenza and Other Respiratory Viruses, a total of 74 mammal species are known to have been infected so far. Mass die-offs have unfolded among sea lions and seals in South America, where about 50,000 of the animals have succumbed. For mammals as well as birds, animals that congregate in large groups are at particularly high risk, says Plaza, the report’s lead author. Because some studies suggest the virus persists longer at lower temperatures, wildlife that live in cooler regions also may be more vulnerable, he adds.

Ultimately, “we’re dealing with a serious virus that can infect both mammals and birds very efficiently, and that’s a scary perspective,” says Ralph Vanstreels, a wildlife veterinarian at the University of California, Davis, who in 2023 was part of a team that witnessed a catastrophic die-off of southern elephant seals on Argentina’s Península Valdés. The outbreak began during the September breeding season, with the virus likely spreading rapidly from mothers to their young, and through saliva and nasal discharge animals spray at each other during territorial rivalries.

Soon, the beaches were covered with dead seal pups—more visible than adults whose heavy bodies tend to sink in water. “Pups are so cute. They have those big eyes. They’re just packets of joy. And seeing them dead was really … difficult,” Vanstreels recalls. He and his colleagues estimate that 95 percent of that year’s pups—more than 17,000 individuals—died, a major setback that will take the population decades to recover from.

The following year, even remote Antarctica saw its first confirmed H5N1 wildlife deaths. Because the two most impacted animal groups—birds and sea mammals—congregate during the continent’s summer, scientists had long feared an outbreak on the Antarctic peninsula. To assess the damage, in early 2024 and 2025, several teams of scientists—including Vanstreels—donned hazmat-type suits to search for carcasses, extracting brain and lung samples to test for H5N1. In both years, they discovered multiple sites with dead skuas, a kind of predatory seabird. These probably are just a fraction of deaths occurring across the vast, unexplored continent, says Vanstreels.

Scientists worry that the impacts of such die-offs could be significant not just for wildlife but also for people and entire ecosystems worldwide. For example, dead birds mean less of their poop, or guano, an important fertilizer in both natural and agricultural systems. Fewer animals also equals fewer tourists, an important source of income for national parks and the people who work for them. And the loss of scavenger species, which play an important role in cleaning up diseased carcasses, could cause more disease to spread to more animals. “You could have problems down the line … because the skuas are not there to clean up,” Vanstreels says.

An image of a California condor receiving a vaccination.

In Arizona’s Vermilion Cliffs National Monument, a California condor receives a bird flu vaccine, one of 42 condors in the population vaccinated by late 2025.

Successful vaccine trials

Despite continuing bad news about H5N1’s spread, signs of hope have emerged. For a few impacted species, such as California condors, elephant seals and African penguins, scientists are testing vaccines, including those developed to protect poultry against the virus. Like human vaccines for flu and other infectious diseases, these immunizations entail injecting animals with weakened, modified or fragmented versions of the virus that do not cause infection but stimulate the immune system to produce protective antibodies if an animal encounters the real thing.

In 2023, when bird flu began infecting condors, scientists from the U.S. Fish and Wildlife Service, U.S. Department of Agriculture and U.S. Geological Survey (USGS) started discussing adapting poultry vaccines for the endangered birds. First, they tested the two-shot vaccination on closely related but more abundant black vultures to ensure it was safe for condors, recalls Todd Katzner, a research wildlife biologist with USGS.

After encouraging results with the vultures, Katzner and his colleagues tested the vaccine on 20 captive condors in zoos. Although the condors did not develop as many antibodies as the vultures had, Katzner says the trial was “highly successful.” In September 2024, officials began vaccinating condors in breeding facilities as well as some wild birds being treated in captivity for lead poisoning or injuries. By October 2025, 42 wild condors in the Arizona–Utah population had received at least one dose of the vaccine as had dozens more in the California–Oregon and Baja California, Mexico, populations along with more than 100 captive condors.

An image of a collared male bobcat with an orange ear tag.

Several bobcats in New York state have developed natural immunity to the virus.

The promise of natural immunity

Vaccines are not a viable solution for most affected species, however. “If you imagine the number of wildlife out on the landscape, it’s virtually impossible” to vaccinate every individual, says Cornell University wildlife veterinarian Jennifer Bloodgood. Fortunately, there’s growing evidence that many animals survive H5N1—and develop natural antibodies that may help protect them if they encounter the virus again.

In New York state, as part of a research project on bobcat health, Bloodgood has been analyzing the blood of wild bobcats to look for H5N1 antibodies, a sign they’ve been exposed and survived. Between October 2023 and March 2025, she and her colleagues tested 24 individuals and found that four did have H5N1 antibodies. While one other bobcat died from the virus, “we’ve got these animals that are mounting an immune response and surviving,” Bloodgood says.

Stallknecht and his colleagues have conducted similar blood testing surveys for dozens of North American bird species, from ducks and geese to raptors and seabirds. “In almost everything we’ve looked at, there is a significant proportion of tested birds that have antibodies and are clinically normal and have probably survived,” he says. That includes nearly all blue-winged teal Stallknecht tested in the Mississippi flyway, almost all double-crested cormorants in the Chesapeake Bay and 60 percent of bald eagles in the upper Midwest.

But how long will either natural immunity or vaccines be effective against a virus that is continually changing and evolving? One reason people receive a new flu vaccine each year is that levels of flu-targeting antibodies decline over time. Another reason is that flu viruses, like H5N1, can change so much in a short time that the previous year’s antibodies become less effective.

How many H5N1 animal deaths we’ll see in the future—and what that means for wildlife populations—is hard to say, notes infectious disease expert Richard Webby of St. Jude Children’s Research Hospital. “We’re not smart enough to know exactly how to predict it.”

Another unknown is the impact the virus might have on humans in the future. More than 900 people worldwide are known to have been infected by H5N1 since 2003—including dozens who work with poultry or cattle in the United States—and hundreds have died globally. But it remains a mystery whether the virus could further evolve and spawn large, sustained outbreaks in human populations. Among scientists, “the opinions there range from ‘it’s very unlikely’ to ‘it’s likely.’ I think that illustrates the uncertainty we have about that,” Webby says.

Fortunately, for Arizona’s endangered condors, the 2023 outbreak left about half of 50 unvaccinated birds tested with protective levels of antibodies. Though Hauck and his colleagues will remain vigilant, it gives him hope that so many birds survived H5N1.

That includes condor 982, who ultimately was nursed back to health by veterinarians at Liberty Wildlife in Phoenix. Today, it’s as if nothing ever happened, Hauck says. “She’s been just a normal wild condor doing condor things—flying throughout the range of northern Arizona and southern Utah and feeding great.” Now eight years old, the bird has reached breeding age and hopefully will soon find a mate. “We’re going to hope that she pairs up and can be one of those great contributors to increasing the population numbers,” he says.

Katarina Zimmer is a science and environmental journalist who is based in Berlin.